1548
J . Phys. Chem. 1984, 88, 1548-1554
which was pumped by an N R G 0.7-5-200 pulsed-N, laser (0.7M W peak power); the dye laser output was passed through a 620-nm interference filter. The detection system consisted of the Aminco-Bowman spectrometer equipped with a Hamamatsu R928 PMT whose output across a 50-0 to ground resistor was measured by a Tektronix Model 466 storage oscilloscope. Decay curves were obtained by operating the oscilloscopein variable-persistence mode which averaged several hundred pulses. A pulse rate of -20 s-’ was used. Peak laser intensities, estimated as described previously,’* of -25 and 3 kW/cm2 were used with no observable difference in results. PL was detected from -680 to 740 nm (-5-nm bandwidth) in both polarizer orientations and in the absence of a polarizer. A combination of Corning 2-64 and Melles Griot 03-FCG-055 filters was placed before the PMT in these experiments. PEC and EL Experiments. Potentiostatic iLV curves were obtained as a function of polarizer orientation with cellsI6 and
electrochemical equipment’ previously described. Both 5 14.5- and 632.8-nm excitations were used for these experiments with the laser beams expanded and masked to fill the electrode surface. Incident-light intensities were measured with a Tektronix J16 radiometer equipped with a 56502 probe head by reassembling the cell outside the Aminco-Bowman spectrometer used in determining iLV data.16 E L spectra as a function of polarizer orientation were obtained in N,-purged peroxydisulfate electrolyte by using a pulse technique previously described.’s2 The experimental geometry differed from that shown in Figure 1 in that the weak EL signal required placement of the emitting electrode directly in front of the emission detection optics.
Acknowledgment. We are grateful to the Office of Naval Research for support of this work. A.B.E. gratefully acknowledges support as an Alfred P. Sloan Fellow (1981-1983). Registry No. CdSe, 1306-24-7;S202-, 15092-81-6;Se?-, 25778-65-8.
Raman Spectroscopic Study of Low-Temperature Phase Transition and Molecular Motion in Urea Clathrates J. Le Brumant, Laboratoire de Physique MolPculaire, UniversitP, Pierre et Marie Curie, 75230 Paris Cedex OS, France
M. Jaffrain,* and G. Lacrampe Laboratoire de Physique MolPculaire Biologique, U.E.R. d’Etudes MZdicales et Biologiques, UniversitZ Rene Descartes, 75270 Paris CPdex 06, France (Received: June 13, 1983)
It is known from specific heat measurements and crystallographic studies that urea inclusion compounds present a phase transition below room temperature, the nature of which is essentially related to the reorientation of the long-chain alkane included molecules, as has been shown by dielectric and NMR experiments. We present here a Raman spectroscopic study of such compounds (cetyl alcohol and lauryl alcohol as guest molecules) in the 20-3500-cm-’ range, and from 300 to 80 K. The decrease of temperature from room temperature brings about important and gradual modifications in the whole spectrum of the urea lattice as well as for the trapped molecules. It appears that, if the lattice vibration modes of the urea present no discontinuity when passing the temperature transition, strong effects are noticed in the C-H stretching vibration region of the alkane included molecules. Evidence of the transition is given by plotting the “intensity ratio” of the 2850and 2885-cm-’ peaks, Le., the stretching vibrations of the methylene groups, as a function of the temperature. The results thus obtained have been interpreted in terms of modifications in the motion of alkane molecules. The sensitivity of this spectral region to structural changes is usually employed for phospholipid and membrane fluidity studies. In the clathrates, the intermolecularinteractions between the chains are negligible, so their study offers the possibility of identifying the intramolecular contribution to these order-disorder transitions.
Introduction It is well-known that urea forms crystalline addition compounds with a great variety of aliphatic straight-chain molecules containing more than six carbon atoms. The resulting complexes have been variously termed urea inclusion compounds, urea adducts, or clathrates. * At room temperature, long needlelike crystals, hexagonal in cross section, are obtained. X-ray detailed structure investigation~*-’~ showed that the unit cell is hexagonal and that the lattice ~~~
(1) Fetterly, L. C. In “Nonstoechiometric Compounds”; Academic Press: New York, 1964; p 496. (2) Schlenk, W., Jr. Ann. Chem. 1949, 565, 204. (3) Smith, A. E. Acta Crystallogr. 1952, 5, 224. (4) Lenne, H. U. Z . Kristallogr. 1961, 115, 297. (5) Lenne, H. U. Z . Kristallogr. 1963, 118, 454. (6) Lenne, H. L. Z . Kristallogr. 1963, 118, 468. (7) Siemons, J. L. ThEse d’Etat, Paris, 1971. (8) Radell, J.; Hunt, P. D. J . Am. Chem. SOC.1958, 80, 2683. (9) Geiseler, G.; Richter, P. Chem. Eer. 1960, 93, 2511. (10) Radell, J.; Brodman, B. W.; Bergman, E. D. Tetrahedron 1963, 19, 813. (1 1) Radell, J.; Brodman, B. W.; Bergman, E. D. Tetrahedron 1964,20, 13. (12) Radell, J.; Brodman, B. W.; Bergman, E. D. Can. J . Chem. 1964,42, 1069.
0022-3654/84/2088-1548$01.50/0
parameters are independent of the guest molecule. The urea molecules form a hollow channel structure in which the n-paraffin molecules, in loose interaction with the host lattice, are enclosed. The hexagonal compounds undergo an endothermal decomposition below the melting point of rea'^,'^ with a change in the crystal structure of urea which transforms to its normal tetragonal form. The aliphatic molecules are in an extended planar zig-zag configuration with their long-chain axis parallel to the c axis; the molecular planes are randomly distributed over positions perpendicular to the a axis and at multiples of 60° to this position. Evidence of the dynamic nature of the trapped-molecule disorder comes from dielectric r e l a x a t i ~ n ’ ~and - ~ ~nuclear magnetic resoLaves, F.; Nicolaides, N.; Peng, K. C . Z . Kristallogr. 1965, 121, 258. Brodman, B. W.; Radell, J. Sep. Sci. 1967, 2, 139. McAdie, H. G. Can. J . Chem. 1962, 40, 2195. McAdie, H. G. Can J . Chem. 1963,41, 2144. Meakins, R. J. Trans. Faraday SOC.1955, 51, 953. Lauritzen, J. I. J . Chem. Phys. 1958, 28, 118. Jaffrain, M.; Siemons, J. L. C. R . Hebd. Seances Acad. Sei. 1968, 266, 1323. (20) Jaffrain, M.; Schuster, P. C. R. Hebd. Seances Acad. Sei. 1968, 261, 1011. (21) Jaffrain, M.; Dansas, P.; Sixou, P. J . Chim. Phys. Phys.-Chim. Biol. 1969, 66, 841.
0 1984 American Chemical Society
Phase Transition in Urea Clathrates
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1549
n m r measurements; the long-chain molecules undergo restricted rotations with a potential barrier of 4-8 kJ mol-'. At low temperatures, heat capacity measurements were performed on hydrocarbon adducts and showed at least one region Y13K of abnormal high heat a b s o r p t i ~ nat~ ~a ,temperature ~~ depending on the molecular length. Dielectric relaxation, depolarization thermocurrents,22 and NMR25-27 studies suggested that this anomaly may be related to an order-disorder transition of the included molecules. More recently, Chatani et obtained the low-temperature X-ray structure of these adducts and gave evidence of a phase transition from the hexagonal structure to an orthorhombic one. The transition can be regarded as an order-disorder transition with respect to the guest molecules with a cooperative deformation of the urea channels resulting in the orderly orientation of the Figure 1. Raman spectrum of urea/cetyl alcohol clathrate at 300 K guest molecules below the transition. (high-temperature form). Previous experimental results obtained in our laboratory had shown the interest of vibrational spectroscopy for this p r ~ b l e m . ~ ' ? ~ ~ The spectrum of these compounds is dominated by the urea vibration modes; the only large region completely free from host molecule vibrations extends from 2000 to 3000 cm-'. That may explain the weak attention paid so far to guest molecules. The foregoing authors interested in urea have principally specified the assignments of the hexagonal urea spectrum in relation to that of tetragonal urea. It appeared that the Raman spectrum of the C-H stretching vibrational region (3000-2750 cm-l) is very sensitive to structural changes and intermolecular interactions between the As the alkane chains in the urea clathrates are in such an environment that interchain interactions are different from those observed in alkane lattices, it was interesting to look more precisely k ; > : / / / t t t . t 3600 3400 3200 3002 2800 1100 900 700 500 300 100 C m 1 at this spectral region. A paper by Snyder et al.45has shown the importance of such a study. The IR vibrational structure of this Figure 2. Raman spectrum of urea/cetyl alcohol clathrate at 80 K (low-temperature form). frequency range seemed to be insensitive to the temperature. Nevertheless, recent results obtained with a Fourier transform We present here a study of such compounds by Raman diffusion IR spectrometer show the possibility of also obtaining information in the 300-80 K range-host lattice as well as included from absorption ~ p e c t r a . ~ ~ . ~ ~ molecules-in order to get indications about the nature of the low-temperature transition. ~
(22) Dansas, P.; Sixou, P.; Jaffrain, M. Mol. Phys. 1971, 21, 225. (23) Kromhout, R. A.; Moulton, W. G. J . Chem. Phys. 1955, 23, 1673. (24) Gilson, D. F. R.; McDowell, C. A. Nature (London) 1959,183, 1183. (25) Gilson, D. F. R.; McDowell, C. A. Mol. Phys. 1961,4, 125. (26) Bhatnagar, V. M.; Nakajima, H. Chem. Ind. 1965, 38. (27) Umemoto, K.; Danyluk, S. S. J. Phys. Chem. 1967, 71, 3757. (28) Pemberton, R. C.; Parsonage, N. G. Trans. Faraday SOC.1965,61, 2112. (29) Parsonage, N . G.; Pemberton, R. C. Trans. Faraday SOC.1967,63, 311. (30) Chatani, Y.; Anraku, H.; Taki, Y. Mol. Cryst. Liq. Cryst. 1978, 48, 219. (31) Brion, D. Thbe de 38me cycle, Paris, 1972. (32) Brion, D.; Jaffrain, M. C. R. Hebd. Seances, Acad. Sci., 1972, 272, 989. (33) Verrijn Stuart, A. A. Recl. Trav. Chim. Pays-Bas 1956, 75, 906. (34) Mecke, R.; Kutzelnig, W. Fresenius' 2.Anal. Chem. 1959,170, 114. (35) Barlow, G. B.; Corish, P. J. J . Chem. Soc. 1959, 1706. (36) Durie, R. A.; Harrisson, R. J. Spectrochim. Acta 1962 18, 1505. (37) Radell, J.; Brodman, B. Can. J. Chem. 1965, 43, 304. (38) Gosavi, R. K.; Rao, C. N . R. Indian J . Chem. 1967, 5, 162. (39) Brion, D.; Garson, J. C. D.E.A. Physique Moleculaire, Paris, 1970. (40) Fawcett, V.; Long, D. A. J . Raman Spectrosc. 1975, 3, 263. (41) Gaber, B. P. Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260. (42) Wallach, D. F. H.; Verma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153 and quoted references. (43) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta Part A 1978, 34, 395. (44) Fainman, R.; Long, D. A. J . Raman Spectrosc. 1975, 3, 379. (45) Snyder, R. G.; Scherer, J. R.; Gaber, B. P. Biochim. Biophys. Acta 1980, 601, 47.
~
~
~
Sample Preparation The studied compounds (urea/cetyl alcohol clathrate, urea/ lauryl alcohol clathrate) were obtained by recrystallization of urea and the corresponding alcohol in ethanol. The sample quality was checked in the following way: (1) by DTA measurements of the temperature and enthalpy of decomposition; our results are in good agreement with the foregoing determination of McAdie; l 6 (2) by the absence of the C-N stretching vibration band at 1000 cm-' in the Raman spectrum; this vibration is observed at 1025 cm-' in the hexagonal form.34 Experimental Conditions The Raman spectra were excited by the 5145-i% line of a Spectra-Physics argon ion laser, using a maximum power of 300 mW. The spectra were recorded by a Jobin-Yvon spectrograph, type Ramanor HG 2s. An interferential filter ruled out the plasma bands up to 130 cm-' from the Rayleigh band. The spectra were recorded in the 20-3500-cm-' range. The spectral slit widths were 2 cm-' for the strong bands and 5 cm-I for the weaker bands. The samples, as a fine powder, were sealed in glass capillary tubes (inner diameter: 1 mm). For measurements between 300 and 80 K, we used a Coderg cryostat, type Cryocirc, and, above 300 K, a Eurotherm furnace. The spectra were recorded for every 10 OC decrease in temperature; the temperature was stabilized more than 0.5 h before each (46) MacPhail, R. A,; Snyder, R. G.; Strauss, H. L. J. Chem. Phys. 1982, 77, 1118.
(47) Cameron, D. G.; Mantsch, H. H. Biochem. Biophys. Res. Commun. 1978, 83, 886.
Le Brumant et al.
1550 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 TABLE 1 u: cm”
300 K -3450 \ h -3405 sh 3380 s 3310 w 3270 w 3226 s
80 K
-3410 sh 3358 s 3314 w 3268 w 3225 s -3200 sh 2955 vw, b 2956 vw, b 2935 w 2935 fi -2900 sh -2905 sh 2881 s 2886 F -2870 sh 2866 w 2845 s 2847 F 2123 vw 2126 vw 1707 w 1715 w -1700 sh 1706 sh 1681 w 1689 w 1663 w 1668 w 1656 sh 1662 w 1597 vw 1595 vw 1563 m -1570 vw 1557 m 1554 m 1514w 1527 w 1478 vw, s h 1474 vw 1455 w , sh 1460 w 1435 in 1433 m 1372 vw 1373 vw 1297 in 1294 in -1180vw,sh 1177vw -1145 sh -1 145 sh 1131 m 1 1 32 in 1 I 03 v w 1102 vw I080 v w , sh I080 vw, sh 1065 in 1065 in I032 vs :,sh 1022 w IO13 vw, sh 8 9 3 vw 895 vw 873 vw -880 VH‘ 795 v w 802 vw -726 v w , b 728 vw 630 vw 610 in 1616 m 553 vw 1537 m -203 vw? sh 165 s 186 5 1 32 in 139 in 109 sh 97 vs 101 vs 5 9 w, b 62 vw 55 vw ~
assignments NH, NH, NH, NH, NH,
stretching stretching stretching ytrctching stretching
(A, g) (A, g)
(E, g) (a, and b,) (Alg) (A, g)
CH, stretchingc CH, stretchingC CH, asyin stretchingL CH, sym stretchingc
~~
-
~-~~~~{
1 12;;
:;
NH, bending(AIg) (a,)
NH, bending (E, g) (a, 1 NH, bending (Elg) (b,) NH, bending ( E l g ) ( a , ) C-0 stretching ( A l g )(a,) C-N stretching ( E l g ) (b,) cH, bending
1
CH, bending NH, rocking ( A , g ) ( a , ) C-C stretching CH, bending NH, rocking (Elg) (b,) C-N stretching (A, ( a , ) CH, bending
s k e l e t a l d e f ( E I E )(b,)
skelctaldef ( A l g ) ( a , ) “accordion mode” (’?) lattice mode (Ezg) lattice mode (A, E) lattice mode lattice mode ( E l g ) lattice mode ( A l E ) lattice mode
a s = strong; in = medium; w = weak; sh = shoulder: v = very: b = Froin Fawcett and Long4’ for hexagonal urea Raman broad. frequencies rccorded at room temperature; assignments based on a fnctor rroupD,h. Generally agreed assignment.
recording. We checked that such stages allowed the sample to attain thermal equilibrium.
Experimental Results The vibrational spectra of the urea molecules are the same whatever the guest molecules may be. Typical spectra of the cetyl alcohol clathrate at 300 and 80 K are displayed in Figures 1 and 2. Table I lists the frequencies of the observed bands. The proposed assignments are those of Fawcett and Long.40 They have not been reinvestigated in the present study. As indicated before, most of the observed bands are to be assigned to the urea vibration modes. Important modifications of the spectra appear between 300 and 80 K, especially in following ranges: 3500-3100, 1050-950, 650-500, and 300-20 cm-’. We note apparent intensities and half-bandwidth changes in the whole spectrum, important fre-
quency shifts in the low-frequency region and in the hydrogenbonded N-H stretching frequency range. If the guest molecule vibrations may be principally observed in the C-H stretching region (3000-2750 cm-’), we also have noticed between 1500 and 700 cm-’ many weak bands which may be assigned to CH2 deformation or rocking vibrations and to skeletal optical modes of guest molecules.
Discussion Urea Lattice. The urea molecule is planar and belongs to the point group, Cb.Its 18 internal vibrations are distributed among the symmetry classes of the C2, group as follows: rurea = 7 a, 2 a2 + 3 bl + 6 b2
+
At room temperature, a full factor group treatment of the P6J2 lattice with six molecules per unit cell and a D6 factor group gives 141 optical vibrations (108 internal vibrations and 33 external vibrations), 105 of which should be Raman active with the following classification: F,(Ra) = 9 AI + 18 El + 18 E2 r,(Ra) = 2 A,
+ 5 El + 6 E2
The E, and E, vibrations being doubly degenerated, a maximum of 45 bands of internal vibrations and 13 external vibration bands are to be expected. The observed spectrum is far simpler than the theoretical one, so, as the urea molecule is nearly in a site of symmetry C2”,the approximate treatment of a Dbh factor group has been ~ s e d . In ~ this ~ j ~ case, ~ we obtain for the Raman-active bands ri(Ra) = 7 A,, + 8 E,, + 10 Ezs r,(Ra) = 1 A,,
+ 3 El, + 3 E2,
At low temperature, a D2 factor group corresponds to the P212121 ( D i ) space group observed by ChatanL30 This space group contains only one set of sites of symmetry Cl with four molecules per site.48 So, all the optical vibrations should be Raman active, and we obtain I?, = 18 A 18 B1 18 B2 18 B3
+ + + re = 6 A + 5 B, + 5 B, + 5 B,
In view of this, a more complicated spectrum is expected at low temperature compared with the room-temperature one. Nevertheless, the number of observed bands is fewer than expected, even at room temperature. Polarization studies on oriented single crystals could bring us further information. Fawcett and Long40 did perform such measurements at room temperature, but there seem to be some contradictions between their results and what could be expected from group theoretical considerations. Some comments can be made from a comparison between room-temper ature and low-temperature spectra. 3500-31 00-cm-l Frequency Region. Here we find the N-H stretching vibration modes. They are termed antisymmetric (u,) or symmetric ( u s ) modes according as the vibrations in a given NH2 group are out of phase or in phase. In the isolated molecule (symmetry C2,), the four N-H stretching vibrations are then classified into antisymmetric (one a, + one b2) and symmetric ones (one a l + one b2). In the clathrate spectrum, we note that the apparent intensity of the band observed at 3310 cm-’ at 300 K decreases with the temperature, whereas the 3226-cm-’ band strongly increases when going from 80 to 300 K (Figures 1-3). When the urea is trapped in an argon matrix,49only two bands are observed: u, = 3548 cm-’ and us = 3440 cm-’. This result is not very suprising if we compare these values with the u(N-H) frequencies in acetamide: for the acetamide molecule, which contains only one NH2 group, King49obtained in an argon matrix U, = 3557 cm-’ and us = 3436 cm-’. This is evidence of low (48) Turrel, G. In “Infrared and Raman Spectra of Crystals“; Academic Press: London, 1972; p 340. (49) King, S. T.Specrrochim. Acta, Part A 1972, 28, 165.
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1551
Phase Transition in Urea Clathrates
L
1030
I\
t L
'$0
1025
-
3 100
140
180
220
260
300
TABLE I1 v , cm-'
tetragonal ureaa hexagonal ureaa a
Unpublished results.
300 K
80 K
Av
3433 3380
3423 3358
10 22
Urea/cetpl alcohol clathrate.
intramolecular coupling between the two NH2 groups of the isolated urea molecule. A similar result was observed in solution:50 v, = 3500 crn-', v, = 3385 cm-'. In tetragonal as well as in hexagonal urea, the molecules form a hydrogen-bonded network with different lengths of the hydrogen bond for each bond of a given N H 2 group. It is well-known that the frequencies of the N H vibration modes strongly depend on the length of the hydrogen bond: the shorter the length of the bond, the greater is the shift in f r e q ~ e n c y . ~In ' tetragonal urea, the two hydrogen bonds are of similar lengths (2.99 and 3.03 and the frequency shifts are of the same order for v, and v, (- 100 cm-' if compared to matrix values); so, it is difficult to put forward a modification in the intramolecular coupling. On the other hand, in hexagonal urea, the hydrogen bonds are more asymmetric: 3.04 A in a direction approximately parallel to the c axis, and 2.93 A in nearly perpendicular directions. This results in a greater shift of v, and v, with regard to the argon matrix values (mean value of Av, = 150 cm-', mean value of Av, = 203 cm-I) and the splitting between the components is also more important (-26 cm-' between the two v, and the two v, modes). It is interesting to note that the greater shift of the v, vibration is in accordance with some calculations of Zhukova and Shmanko; 53 these authors looked at the shift in frequency of v, and v, in the two following cases: only one of the N H bonds makes a hydrogen bond with another molecule, or the two NH bonds are hydrogen bonded in a similar manner. They found that the v, frequency shift is more important than the v, one when only one vibrator is perturbed. In the case of hexagonal urea, the perturbation is asymmetric, when it is nearly symmetric in the tetragonal urea. So, the asymmetry of the intermolecular coupling is likely to introduce a greater splitting in the v, and v, modes; it is not possible to say here if the observed splitting comes from an intramolecular origin (two NH, groups per molecule) or from an intermolecular one (six molecules per unit cell). Finally, it should be noted that only the highest frequency vibration mode is significantly temperature dependent (Table 11). This could be related to the lattice contraction of the urea channels with lowering of the temperature. Using a well-known expression given by Pimentel and Sederholmsl correlating the v(N-H) fre~~~~~
~~
(50) Kutzelnig, W.; Mecke, R.; Schrader, B.; Nerdel, F.; Kresse, G. Z . Electrochem. 1961, 65, 109. (51) Pimentel, G . C.; Sederholm, C. H. J . Chem. Phys. 1956, 24, 639. (52) Vaughan, P.; Donohue, J. Acta Crysiallogr. 1952, 5 , 530. (53) Zhukova, E. L.; Shmanko, I. I. Opt. Spectrosc. 1968, 25, 279.
/-
: "33
T
Figure 3. Peak height ratio 3266-cm-' band/3280-cm-I band vs. temperature, for urea/cetyl alcohol clathrate.
\
140
180
220
\%
263
300
T
Figure 4. C-N stretching vibration frequency as a function of temper-
ature. quency shift of a bound vibrator with respect to the free one, with the length R of the NH. -0bond, one can estimate the variation AK = A(". e o ) as the temperature varies from 300 to 80 K: AV = ~ 3 0 0- v80 = 548(R80 - R300) = 548AR We thus obtain AR = 0.04 A. This value may be compared with the change in the channel dimensions measured by Chatani et al.: 30 the side of the hexagonal section varies from 4.75 8, at room temperature to a mean value of 4.705 8, at low temperature, Le., AI? = 0.045 8,. The agreement between these two values suggests that the 3380-cm-' band could be assigned to the shortest hydrogen bond, perpendicular to the c axis. 1050-950-~m-~ Frequency Region. In this region there is a very strong band located at 1025 cm-' at 300 K and which shifts toward 1032 cm-' at 80 K (Figures 1, 2, and 4). This band is assigned to the symmetric C-N stretching vibration mode (al) and we noticed that it is a good test of purity for the clathrate, since the presence of any tetragonal urea is indicated by the appearance of the corresponding band at 1000 cm-I. The frequency shift of this band from tetragonal to hexagonal urea at room temperature has been interpreted in terms of a slight shortening of the C-N bond in the hexagonal form. It appears then that there is a further shortening of the bond with the lowering of temperature; a similar shift has been observed in tetragonal urea.31 It may be interesting to notice a weak satellite band on the low-frequency side of the C-N stretching band; it is resolved only below 220 K and appears at 10 cm-I below the main band. An isotopic calculation suggests assigning this band to the 13C-N stretching mode. 650-500-cm-' Frequency Region. There are two bands in this region, associated with the skeletal vibrations; they are respectively found at 533 (symmetric mode, al) and 610 (antisymmetric mode, a2) cm-'. In tetragonal urea, the equivalent bands are located at 556 and 570 cm-1.31,40Once again, the important frequency shift when going from tetragonal to hexagonal urea shows the nonnegligible effect of intermolecular interactions arising from hydrogen bonds on the internal vibration spectrum. We also have observed a relative change of the height of these bands with temperature: the 610-cm-' band is the strongest at 300 K, but becomes the weakest at 80 K (Figures 1 and 2). Yet, at the same time, there appears a variation of the half-bandwidth in the opposite direction (Figure 5), so the product height-width remains nearly constant. Figures 6 and 7 depict the variations of the peak height ratio R and half-bandwidth ratio R' for these two bands. It is noteworthy that these variations essentially result in changes of the a l type band (533 cm-I at 300 K). This band is the most sensitive to intermolecular interactions: it shifts from 504 cm-I for solid solution in CH3CN to 556 cm-' for tetragonal urea, while under the same conditions the b2 type band moves only from 575 to 570 The phase transition being sometimes sluggish, we used this inversion of the relative intensities between 300 and 80 K to verify
1552 The Journal of Physical Chemistry, Vol. 88, No. 8. I984
Le Brumant et al.
!
10
8
-
160
7 -
4
4
I
6 5 I
100
140
180
220
260
300
T
Figure 5. Half-bandwidth(in cm-l) plotted against temperature for (a) the 533-cm-' band and (b) the 61O-cm-' band.
110
-
100
-
---
90 -
R?
100
140
180
220
260
300
T
Figure 8. Frequency shifts vs. temperature for low-frequencyvibrational
t
0,5
region.
J 180 220 260 300 T 100
140
Figure 6. Peak height ratio 610-cm-' band/533-cm-I band plotted against temperature.
R'
0,s
t
Then, from the D6h-D6 correlation, and from the data of Fawcett and Long, we may tentatively assign the high-temperature spectrum as follows: 59 cm-', Al; 132 cm-', Al; 97 cm-', El; 165 cm-', E2. It is interesting to note that the I R spectrum is also simpler than the expected one: when the selection rules give eight bands (three A2 five E, in the D6 group factor hypothesis), we observed in a previous work3' only four bands at 117, 124, 169, and 181 cm-l. We had not noticed an absorption band near 97 cm-I as might be expected, but, at low temperature, a weak band arises near 92 cm-'. With decreasing temperature, only the highest frequency band undergoes a strongly marked shift: 165 cm-' at 300 K, 186 cm-' at 80 K (Figures 1,2, and 8). An analysis of the correlation table points out that this mode results from translation and rotation movements of the urea molecule about the different axis. So, it is not possible to assign this band to a definite motion; at the very most, we may think that this band, sensitive to the lattice contraction when lowering the temperature from 300 to 80 K, mainly corresponds to translations along an axis almost perpendicular to the c axis of the hexagonal or orthorhombic crystal. We point out that when the average value of the hexagon side contracts by 10.5%,30the considered band undergoes a relative shift of 12%. The intensity of the 97-cm-I band strongly decreases with temperature, more particularly between 300 and 200 K. At low temperature there appear three very weak bands at 55, 109, and 203 cm-' (at 80 K); the last one might represent the "accordion mode" of the guest molecule, here cetyl alcohol. In the whole range of temperature, the Raman bands gradually shift toward longer wavelengths and we did not observe any particular inflection near the transition point. These results are in good agreement with data of Chatani et al." which show that the urea lattice undergoes only a weak deformation at the transition point, with a light distortion of the hydrogen-bond network. Therefore, we may consider to a good approximation that selection rules of the D6 group remain available at low temperatures. Guest Molecule. Since the works of Lippert and P e t i c o l a ~ ~ ~ ~ ~ ~
+
i IO0
I i O
180
220
260
300
7
Figure 7. Half-bandwidth ratio 610-cm-l band/533-cm-l band plotted
against temperature. that the transformation really occurred. From room temperature, the 533-cm-' band has a very weak satellite band. A similar band appears also for the 616-cm-' band at low temperature. In both cases, they are pointed 14 cm-' higher than the main band (Figure 2, Table I). 200-20-~m-~ Frequency Region. At room temperature, 4 bands arise below 200 cm-l, when 13 bands were to be expected in the D6 group factor hypothesis, and 7 in the DLhone. The observed frequencies (59, 97, 132, and 167 cm-l, Figure 1) are in good agreement with those noticed by Fawcett and Long.40 The two bands that these authors observed in the b(cc)a spectrum obtained in polarized light (59 and 133 cm-') had been assigned to Al, modes in the hypothesis of a DLhgroup factor. In fact, only one band of Al, type is to be expected in this case. So, it seems better to keep the D6 group factor which foresees two A, bands, the only bands allowed in b(cc)a polarization for the D6 group factor.
(54) Lippert, J. L.; Peticolas, W. L. Proc. Narl. Acad. Sci. U.S.A. 1971, 68, 1752.
Phase Transition in Urea Clathrates
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1553
and L a r s ~ o nnumerous ,~~ studies have been devoted to the examination of phase transitions and conformation changes of longchain molecules by Raman diffusion spectroscopy. Two spectral ranges appear to be particularly sensitive to the conformation and CLATHRATE to the molecular interactions: the region of skeletal optical modes v(C-C), 1000-1 150 cm-I; the region of stretching vibrations v(C-H), 2800-2950 cm-I. a b I VI Skeletal Vibration Region. The skeletal optical modes between 1000 and 1150 cm-’ are particularly sensitive to the conformational state of hydrocarbons. Of the three bands observed in this region in the alkane spectra, the two bands at 1064 and 1 130 cm-’ may be assigned to the all-trans chain segments, while the third, located at 1090 cm-I, results from structures containing “gauche” bond^.^^,^^ In the clathrates, this region is obscured by strong urea bands45and we did not study it. A careful examination of m this frequency range should require urea d e ~ t e r a t i o n . ~ ~ m N C-H Stretching Vibration Region. This spectral range is characterized by a complex group of bands dominated by the stretching vibrations of the methylene groups. The structure of the group results from interactions between v(C-H) vibrations and overtones of CH2 deformations, giving rise to Fermi resonance.43,58,59 Now it is generally agreed that the two strongest bands located at 300 K, at 2886 and 2850 cm-I, can be respectively assigned to the antisymmetric and symmetric vibrations of the methylene groups. The structure of the v(C-H) vibration group proved to be sensitive to the lateral order of the chains, intramolecular deformations, and molecular reorientations of the guest molecules4’ and yields information not only on the temperature of orderdisorder transitions but also on the more or less degree of cooperativity of the transitions. Moreover, it seems that the shape of the v(C-H) bands could provide us with structural information: 3000 2900 2800 Cm-1 3000 2900 2800 cm-I hexagonal, triclinic, or orthorhombic packing of the chains.43 Figure 9. Raman spectra of C-H stretching vibrational region for In the case of urea inclusion compounds, a comparison of the urea/cetyl alcohol clathrate (a) at 300 and (b) 80 K and for pure cetyl spectra recorded at 300 and 80 K gives evidence of a variation alcohol (c) in the liquid phase at 373 K and (d) in the solid phase at 300 in the structure of these compounds between these two temperK. atures (Figures 1 and 2); characteristic of an ordered from a t 80 K, the spectrum clearly shows some type of disorder at 300 K. frequency does not change (v = 2880 cm-I) on getting from the This is emphasized in Figure 9, which depicts the 3000condensed phase (solid alcohol) to the isolated trans molecule 2800-cm-’ region for urea/cetyl alcohol clathrate at 300 and 80 (clathrate at 80 K) (Figure 9, d and b), but it varies on going to K, and for the pure cetyl alcohol in the liquid and solid phases. the disordered form: we notice a gradual shift Av = 5 cm-’ for A comparison of Figure 9b (clathrate at 80 K) and Figure 9d (solid the clathrate between 80 and 300 K, and a shift Av = 20 cm-’ alcohol) shows that, if the 2880-cm-’ band is very sharp in these for pure cetyl alcohol from the solid phase to the liquid phase two cases, it is not the same for the band located near 2845 cm-’; (Figure 9, d and c). this band is much sharper in the clathrate spectrum at 80 K than In order to specify the nature of the transition, we plotted the in the solid alcohol one. The shape of the v(CH,) group in the peak height ratio p ( T ) = 12886/12850 as a function of the temsolid alcohol spectrum is characteristic of an orthorhombic f0rm.4~ perature (Figure 10). The curve thus obtained is typical of the From the X-ray data of Chatani et al.,30this should be the same existence of an order-disorder transition. It presents a flex point for the clathrate at 80 K. That the band observed at 2845 cm-’ generally assigned to the temperature of transition, observed at is sharper than expected is to be related to the fact that, in the 150 K for the urea/cetyl alcohol clathrate (Figure loa), and at clathrate, the guest molecules are isolated from each other. Snyder 180 K for the urea/lauryl alcohol one (Figure lob). In the case et al.43 observed a similar feature for the 1 H / 2 0 D n-C36H74 of urea/cetyl alcohol, the transition occurs at nearly the same Raman spectrum at 110 K. In the same way, these a ~ t h o r s ~ ~ ,temperature ~~ as in the urea/hexadecane complex (specific heat noticed that the band at 2880 cm-’ is less sharp in the hexagonal measurements of Parsonage and Pemberton,28 Figure loa’). This form than in the orthorhombic one. That is what we have observed temperature is also in good accordance with what we previously for this band in the clathrate spectrum: the 2880-cm-’ band is obtained in our laboratory by differential thermal analysis3’ and, sharp at low temperature, broader at 300 K in the hexagonal form more recently, by differential scanning calorimetry. In both cases, (Figure 9, a and b). the specific heat of transition has nearly the same value: 1.38 Evidence of the disordered character of the high-temperature J g-’ for the urea/hexadecane complex,28 and 1.46 J g-’ for form of the clathrate is sketched by analogy with Figure 9a (cetyl urea/cetyl alcohol clathrate (our DSC result). This suggests that alcohol clathrate at 300 K) and Figure 9c (liquid cetyl alcohol). the compounds have almost the same structure. This hypothesis is supported by the frequency shift of the asymThe less pronounced character of the transition for the metric C-H stretching vibration mode of methylene groups.41 Its urea/lauryl alcohol clathrate (Figure lob) may be due to differences in the intermolecular interactions in the two clathrates. With a chain length which is twice the repeat distance of the (55) Lippert, J. L.; Peticolas, W. L. Biochim. Biophys. Acta 1972, 282, hexagonal urea l a t t i ~ e the , ~ urea/cetyl alcohol clathrate seems 8. to be a special case of urea/alkane clathrate. This point has been (56) Larsson, K. Chem. Phys. Lipids 1973, 10, 165. Cameron, D. G.; Casal, H. L.; Compton, D. A. C.; (57) Snyder, R. G.; also noticed by MacPhail et a1.& when studying the CH, vibrations Mantsch, H. H. Biochim. Biophys. Acta 1982, 684, 11 1. of these compounds. (58) Schachtschneider, I.; Snyder, R. G. J . Polym. Sci., Part C 1963, 7, The transition has been assigned by Pemberton and Parso99. nage28,29to a change from a form in which the guest molecules (59) Gall, M. J.; Hendra, P. J.; Peacock, C. J.; Cudby, M. E. A,; Willis, H. A. Spectrochim. Acta, Part A 1972, 28, 1485. do not rotate to a form in which the molecules can statistically
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Le Brumant et al. the peak height ratio for 2880- and 2845-cm-’ bands is 1.45 for the clathrate and 2.13 for the pure cetyl alcohol. In this last case, the ratio is ranging about the observed value for solid hexadecane.4’ The decrease of about 0.6 for the p ratio when going from pure cetyl alcohol in the condensed phase to cetyl alcohol trapped in urea is consistent with the hypothesis expressed by Gaber and Peticolas4’ for a contribution of 0.7 in the ratio, corresponding to the chain-chain lateral interactions in the solid phase. As a matter of fact, the interactions between the guest molecules belonging to two adjacent channels in the clathrate are practically negligible.55 Then, the observed value p = 1.55 would show that, a t 80 K, the cetyl alcohol chains are in the trans configuration. On the other hand, if we compare the urea/cetyl alcohol clathrate spectrum at 300 K with the liquid cetyl alcohol one (Figure 9a and c), we ascertain that, in both cases, the apparent intensity of the 2850-cm-’ band is higher than that of the band located near 2885 cm-’ ( p < 1). This inversion of the apparent intensity ratio is characteristic of a disordered f ~ r r n . ~ ’ , ~ ~ The p value that we have observed for the clathrate at 300 K-of the order of 0.9-is only weakly higher than the residual value in the liquid phase: pliquld E 0.7.41 This result suggests that the reorientation of the guest molecule from an equilibrium position to the other is not going on by “jumps” of the “rigid” molecule (in the trans form), but from deformation of successive segments of the molecule by some sort of twisting. This is in agreement with the gradual frequency shift observed between 300 and 80 K for the v,(C-H) vibration mode, and with Snyder’s considerations on the breadth of the C-H bands.45
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Figure 10. Peak height ratios plotted as a function of temperature: (a) 2886-cm-I band/2847-cm-I band for urea/cetyl alcohol clathrate, (b) 2886-cm-’ band/2847-cm-I band for urea/lauryl alcohol clathrate, (c) 2935-cm-’ band/2847-cm-’ band for urea/lauryl alcohol clathrate, (d) 2935-cm-’ band/2847-cm-I band for urea/cetyl alcohol clathrate; (a’) heat capacity C, (cal deg-l mol of urea)-’) vs. temperature for urea/ hexadecane clathrate; curve plotted for comparison with curve 10a from experimental data of Pemberton and Parsonage (ref. 28, Table IV).
occupy the six equilibrium positions giving to the lattice its hexagonal symmetry. Yet it seems that the transition is not very cooperative, as indicated by the shape of the curves p( T ) (Figure 10a,b): p does not vary in an abrupt way in the vicinity of the transition. This hypothesis is confirmed by the data of dielectric relaxation which point out nonnegligible reorientations of polar groups below the thermodynamic transition, whether the polar group is at the end or in the middle of the chain; lFZ2 on the other hand, the N M R data of Gilson and M ~ D o w e l and l ~ ~Umemoto and D a n y l ~ kalso ~ ~express a gradual increasing of the mobility of the guest molecules. As aforesaid, the general appearances of the groups of bands corresponding to the clathrate at 80 K (Figure 9b) and to the alcohol in the solid phase (Figure 9d) are similar; the value of
Conclusion The results obtained by Raman diffusion spectroscopy for urea clathrates bring out that, if the low-temperature phase transition observed in these compounds represents the transformation from a hexagonal form to an orthorhombic one, this transformation is accompanied not by a drastic reorganization of the urea molecule lattice, but rather by a gradual deformation of it. All of the data obtained on the lattice vibration spectrum as well as on the vibrations of the guest molecules are in favor of a weakly cooperative order-disorder transition, connected to possibilities of reorientation of the guest molecules. The application to this type of compound of a process of analysis (determination of the apparent intensities p ratio of the 2880- and 2850-cm-’ bands), essentially developed in the molecular biology of membra ne^,^^,^^ gave us the means t o specify the nature of movements of the guest molecules: intramolecular deformation accompanying and making the orientation of the whole molecule easier. These results corroborate N M R and dielectric relaxation data. Acknowledgment. We are greatly indebted to Dr. J. L. Siemons (Institut Agronomique, Paris), who kindly provided us with the samples studied. We also thank the SOFRANIE Society for their assistance in DSC measurements on a TA 3000 Mettler apparatus. Registry No. Urea/cetyl alcohol clathrate, 23079-19-8; urea/lauryl alcohol clathrate, 88946-45-6.
